Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine in part due to the engine being shut down during times when the engine operates inefficiently, or is not otherwise needed to propel the vehicle. During these conditions, the vehicle is transitioned from an engine mode to an electric mode where the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine is restarted. Vehicle propulsion is then transitioned from an electric mode to an engine mode.
Vehicle systems, including hybrid electric vehicles, may comprise various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster, a fuel vapor canister etc. Vacuum used by these devices may be provided by a dedicated vacuum pump. In still other embodiments, one or more aspirators (alternatively referred to as ejectors, venturi pumps, jet pumps, and eductors) may be coupled in the engine system that may harness engine air flow and use it to generate vacuum.
Since aspirators are passive devices, they provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. While aspirators may generate vacuum at a lower cost and with improved efficiency as compared to electrically-driven or engine-driven vacuum pumps, their use in engine intake systems has traditionally been constrained by both available intake manifold vacuum and maximum throttle bypass flow. Some approaches for addressing this issue involve arranging a valve in series with an aspirator, or incorporating a valve into the structure of an aspirator. Such valves may be referred to as aspirator shut-off valves (ASOVs) or aspirator control valves (ACVs). An opening amount of the valve is regulated to control the motive air flow rate through the aspirator, and thereby control an amount of vacuum generated at the aspirator. By controlling the opening amount of the valve, the amount of air flowing through the aspirator and the suction air flow rate can be varied, thereby adjusting vacuum generation as engine operating conditions such as intake manifold pressure change.
An example approach of controlling an aspirator control valve (ACV) in a hybrid electric vehicle is shown by Hirooka in U.S. Pat. No. 7,634,348. Herein, the ACV is opened when a controller in the hybrid electric vehicle determines that vehicle motion is primarily due to a motor of the hybrid electric vehicle. To elaborate, motive flow through the aspirator is allowed by opening the ACV when an engine-off condition is determined. Further, the ACV is opened after a pre-determined duration following the engine shutdown command.
The inventors herein have identified potential issues with the above approach to motive flow control in a hybrid electric vehicle. As an example, the hybrid electric vehicle may experience engine shutdown shake due to excessive air flow as the engine comes to rest. Torsional pulses may be caused by pistons compressing and expanding air that is trapped in engine cylinders and these pulses may be transmitted to the vehicle body. Accordingly, engine shutdown events can produce degraded noise, vibration and harshness (NVH), referred to as shutdown shake, a problem that is exacerbated in hybrid vehicle systems as the engine is turned on and off repeatedly during operation of the vehicle. Motive flow through the aspirator as the engine comes to a rest may contribute to NVH due to engine shutdown shake. In another example, oxygen storage in an emission control device may be increased due to air flow via the aspirator after engine shutdown. This increase in oxygen storage content can negatively affect emissions and catalyst performance.
The above issues may be addressed by a method of operating a hybrid vehicle system, comprising, following a shut-down command to an engine, opening an aspirator control valve (ACV) between a first engine speed and a second engine speed, the first engine speed being lower than an idle speed and the second engine speed occurring immediately before an imminent engine stop. In this way, NVH issues due to engine shutdown shake may be reduced.
Another example method for an engine in a hybrid vehicle comprises, following a first shutdown command to the engine, opening an aspirator shut-off valve (ASOV) between a first engine speed and a second engine speed, the second engine speed nominally higher than an engine stop, and following a second shutdown command to the engine, closing or maintaining closed the ASOV irrespective of engine speed.
As an example, an engine system in a hybrid electric vehicle (HEV) may be configured with an aspirator for passive vacuum generation and an emission control device such as a three-way catalyst for reducing emissions. The engine system may be naturally aspirated wherein the aspirator is coupled across an intake throttle in an intake bypass passage. In an alternate embodiment, the engine system may be a forced induction system including an intake compressor. Herein, the aspirator may be coupled in a compressor bypass passage and may route a portion of intake air from downstream of the intake compressor to upstream of the intake compressor. An aspirator shut-off valve (ASOV) may be coupled upstream (or downstream) of the aspirator to vary a motive flow through the aspirator. The ASOV may be opened for additional vacuum generation at the aspirator when a shutdown is commanded to the engine system. Specifically, an opening of the ASOV may be increased to enable motive flow between a first engine speed and a second engine speed. Further, the first engine speed may be lower than an engine idle speed while the second engine speed is higher than an engine speed when an engine stop is imminent. A position of the ASOV following the shutdown command to the engine may also be determined by an oxygen content of the three-way catalyst. If the oxygen content in the three-way catalyst is at or above an oxygen content threshold, the ASOV may be closed or maintained closed irrespective of engine speed being between the first engine speed and the second engine speed.
In this way, additional vacuum may be generated and stored in a vacuum reservoir for future use by opening the ASOV as engine rotation slows down subsequent to an engine-off command in a HEV. However, by closing the ASOV prior to an imminent engine stop, engine shutdown shake may be reduced. Further, by controlling air flow through the aspirator based on the oxygen content stored in the emission control device, the performance of the emission control device may be enhanced. As such, the ASOV may be controlled in a simpler manner that is based on engine speed and oxygen content of the emission control device. Overall, the aspirator is able to meet the brake vacuum demand with improved efficiency without degrading emissions compliance and the vehicle operator's drive experience.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.